Pressure transmitter

ABSTRACT

The invention relates to a pressure transmitter for an overload-proof pressure gauge, comprising a base body, a separating membrane disposed on a pressure-sensitive side of the pressure transmitter and subjectable to a pressure to be measured, a communicating chamber located inside said base body, which receives a pressure transmission medium, said chamber being closed on the pressure-sensitive side by the separating membrane, wherein the coefficient of thermal expansion of the pressure transmission medium is established in such a way that a temperature-induced change in the volume of the communicating chamber equals a temperature-induced change in the volume of the pressure transmission medium.

[0001] The invention relates to a pressure transmitter of the type referred to in the preamble of claim 1.

[0002] Typically, pressure-measuring devices consist of a pressure transmitter and a pressure sensor coupled with the pressure transmitter. The pressure-measuring device is subjected externally via the pressure transmitter to a pressure to be measured that can then be measured by the pressure sensor. The pressure transmitter often serves to protect the extremely sensitive pressure sensor. In the direction of the process medium, the pressure transmitter has a separating membrane that seals off the pressure transmitter externally and which, together with the base body of the pressure transmitter, defines a chamber inside the pressure transmitter in which a pressure-transmitting medium is provided. The materials of the base body and the separating membrane forming the chamber typically have significantly lower coefficients of thermal expansion than the pressure-transmitting medium. Because of this difference in temperature expansion coefficients, temperature changes in the environment lead to differing expansions in the base body, separating membrane, and pressure-transmitting medium. The problem here is that the different temperature-induced volume changes in the chamber and pressure-transmitting medium occasionally caused very large errors of measurement.

[0003] In order to reduce these temperature-induced errors of measurement, pressure transmitters have been designed that have a separating membrane with a comparatively large surface area on the process side. Alternatively or additionally, the volume of the pressure-transmitting medium in the chamber is reduced. However, the volume of the pressure-transmitting medium cannot be reduced at will because of the desired measuring range of the pressure sensor or its measurement tolerances. Moreover, a pressure transmitter with a very small separating membrane diameter is often required so that space considerations alone place limits on the size of the separating membrane in terms of area.

[0004] GM 76 03 126* describes a pressure transmitter according to the species in which an equalizing part with a minimal expansion coefficient is built into the chamber of the base body of the pressure transmitter to compensate for the volume expansion of the pressure-transmitting medium. Such a pressure transmitter is however extremely difficult to produce for manufacturing reasons. Moreover, adjusting this temperature-compensated pressure transmitter, namely exact setting of the proportions of the pressure-transmitting medium and the equalizing body, is extremely expensive. Finally, particularly with pressure gauges for measuring pressures in the millibar range, it is necessary for the pressure transmitter to transmit the pressure received from the outside reliably and evenly to the pressure sensor downstream thereof. However, with the pressure transmitter described in GM 76 03 26^(†) in which the equalizing part floats freely in the chamber, this is not possible or is possible only to a limited extent.

[0005] Hence the goal of the present invention is to provide a temperature-compensated pressure transmitter having the smallest possible separating membrane surface area.

[0006] According to the invention this goal is achieved by a pressure transmitter with the features of claim 1.

[0007] Accordingly, such a pressure transmitter is characterized in that the coefficients of thermal expansion of the separating membrane and the base body and the coefficient of thermal expansion of the pressure-transmitting medium are established such that a temperature-induced change in the volume of the communicating chamber is the same or at least approximately the same as a temperature-induced change in the volume of the pressure-transmitting medium.

[0008] Because of these measures, optimal temperature compensation of the pressure transmitter and hence of the pressure gauge is achieved in a simple manner. The particular advantage of the pressure transmitter according to the invention is that the pressure transmitter head does not have to be enlarged to produce the temperature compensation. Moreover, no additional expensive manufacturing process steps are required. Once a pressure transmitter is specified—namely over the appropriate selection of coefficients of thermal expansion the compensating volume of the separating membrane and the volume increase in the pressure-transmitting medium offset each other—no additional manufacturing steps are necessary when manufacturing the pressure transmitter.

[0009] Advantageously, the separating membrane has a corrugated shape of the radially outer areas of the separating membrane. As the temperature increases, the corrugation of the separating membrane decreases, leading to an increase in volume—the so-called compensating volume. The corrugation allows the compensation to be intentionally set for a given temperature-induced increase in volume.

[0010] The heads of pressure transmitters advantageously have a round cross section of the separating membrane. Of course, separating membranes with other shapes for the pressure transmitter head can also be made, for example oval, hexagonal, or square, but the round shape is by far the most accurate, especially for pressure measurement in the millibar range. Accordingly, advantageous fully compensated pressure transmitters can be produced whose separating membranes have a diameter of less than 40 mm.

[0011] Typically, the base body and the separating membrane are made at least partially of a corrosion-resistant metal material. High-grade steel is preferably used. It would be possible, however, for the separating membrane and/or the base body to be made of a ceramic or ceramic-like material. An oil, for example a hydraulic oil or silicone oil, is typically used as the pressure-transmitting medium. However, any other liquid or even a gas may be used.

[0012] The base body and the separating membrane typically have coefficients of thermal expansion that are far smaller than the pressure-transmitting medium. Usually the coefficient of thermal expansion is in the range of 10-100 10⁻⁶ K⁻⁶ for a temperature range of 20-100° C. and is hence smaller by a factor of 10 to 1000 than that of the pressure-transmitting medium.

[0013] Other advantageous forms and embodiments of the invention appear in the subclaims and the description with reference to the drawing.

[0014] The invention will now be explained in further detail using examples shown in the drawing.

[0015]FIG. 1 is a first embodiment of a pressure transmitter according to the invention; and

[0016]FIG. 2 is a second embodiment of a pressure transmitter according to the invention.

[0017]FIG. 1 is a partial section through a pressure transmitter according to the invention. Numeral 1 is the pressure transmitter of a pressure gauge. Pressure transmitter 1 has a metal pressure transmitter base body 2 preferably consisting of a corrosion-proof oxidation-resistant material. However, base body 2 could consist partially of another material, for example a ceramic material. On the sensor side 3, the base body 2 has a flange 4 that connects the pressure transmitter 1 with a pressure sensor not shown in FIG. 1.

[0018] On the process side 5, pressure transmitter 1 has a separating membrane 6 disposed in a recess 7 in base body 2. Separating membrane 6 is disposed in recess 7 in such a way that a chamber 8 is formed between separating membrane 6 and base body 2. In addition, the base body 2 of pressure transmitter 1 has a hole 9 connected both with chamber 8 and with flange 4. A pressure-transmitting medium, for an oil fill for example, is provided in the communicating chamber 8, 9 consisting of chamber 8 and hole 9. Separating membrane 6 separates the medium on the process side (not shown in the figure) from the pressure-transmitting medium inside the communicating chamber 8, 9. A pressure applied to the process side can be transmitted to the pressure-transmitting medium (and hence to the pressure sensor downstream) via the separating membrane 6.

[0019] The head of the pressure transmitter and hence the separating membrane 6 as well are essentially circular in cross section in the present example. The separating membrane 6 has a corrugated shape at least in the radially outer areas of the separating membrane 6.

[0020] The novel design of the pressure transmitter 1 according to the invention and its operation will be explained in greater detail with the aid of the example:

[0021] The base body 2 of pressure transmitter 1 and separating membrane 6 consist of a material that typically has a much lower coefficient of thermal expansion than the pressure-transmitting medium. With fluctuating temperatures, the separating membrane 6 is compressed or stretched radially. If the temperature rises for example from T=20° C. to T=100° C., the separating membrane 6 is stretched (see dashed line in FIG. 1) so that the volume of chamber 8 increases. If the temperature drops, on the other hand, the separating membrane 6 is compressed, leading to a reduction in the volume of chamber 8. The compensating volume 10 results from the volume difference between the compressed and the stretched separating membrane 6. When the temperatures rise and fall, the pressure-transmitting medium increases and decreases in volume, and so does the above-mentioned oil fill.

[0022] According to the invention, the total volume of the pressure-transmitting medium as well as the coefficients of thermal expansion of the materials of which the base body 2 and separating membrane 6 are made is adjusted such that the temperature-induced change in the volume of the pressure-transmitting medium is the same, or at least approximately the same, as a temperature-induced change in the compensating volume 10. According to the invention, a pressure transmitter 1 is provided that is temperature-compensated over wide ranges, in which the closed hydraulic or pneumatic system of the pressure transmitter 1 is temperature-compensated when the increase in volume, caused by a rise in temperature, of the pressure-transmitting medium—having regard to its special coefficient of thermal expansion—is the same or approximately the same as the volume resulting from an increase in the volume of the communicating chamber 8, 9 having regard to the coefficient of thermal expansion of separating membrane 6 and base body 2.

[0023] The temperature dependence of the pressure transmitter 1 can be reduced to nearly zero by the following measures, which can be taken even in very small measuring ranges of approximately 100 mbar:

[0024] First, the temperature dependence can be adjusted by an appropriate material combination of the base body 2 and separating membrane 6, for example by appropriately choosing their thermal expansion coefficients. Alternatively or additionally, the temperature dependence of the pressure transmitter 1 can be further reduced by a suitable shape of the separating membrane 6.

[0025] In addition, it is especially advantageous for the separating membrane 6 to be concave for example at a low temperature (e.g. T=20° C.) and assume an increasingly convex shape at increasing temperatures (for example T=100° C.). Of course, all other measures for reducing the temperature dependence are possible and advantageous, for example reducing the volume of the pressure-transmitting medium, enlarging the surface area of the separating membrane 6, and reducing the distance between the pressure transmitter 1 and the downstream pressure sensor as far as possible. Thus, in broad ranges, optimum temperature compensation of the pressure transmitter and hence of the pressure gauge can be achieved by these measures.

[0026]FIG. 2 is a section through a pressure transmitter 1 similar to that in FIG. 1. The same parts are designated by the same reference numerals. However, in this case the separating membrane 6 is not corrugated in its outer area, but is straight. Overall, the separating membrane 6, shown once again at T=20° C. (solid line) and at T=100° C. (dashed line) has the shape of a pot.

LIST OF REFERENCE NUMERALS

[0027]1 pressure transmitter

[0028]2 base body (of pressure transmitter)

[0029]3 sensor side

[0030]4 flange

[0031]5 process side

[0032]6 separating membrane

[0033]7 recess

[0034]8 chamber

[0035]9 hole

[0036]10 compensating volume 

1. Pressure transmitter for a pressure gauge, with a base body (2), with a separating membrane (6) disposed on a pressure-sensitive side (5) of the pressure transmitter (1) and subjectable externally to a pressure to be measured, with a communicating chamber (8, 9) inside the base body (2), said chamber accepting a pressure-transmitting medium and being sealed on the pressure-sensitive side (5) by the separating membrane (6), characterized in that the coefficients of thermal expansion of the separating membrane (6) and of the base body (2) and the coefficient of thermal expansion of the pressure-transmitting medium are established such that a temperature-induced volume change in the communicating chamber (8) is the same or at least approximately the same as a temperature-induced volume change in the pressure-transmitting medium.
 2. Pressure transmitter according to claim 1, characterized in that at least part of the separating membrane (6) has a corrugated shape, the corrugation of the separating membrane (6) decreasing with increasing temperature.
 3. Pressure transmitter according to one of the foregoing claims, characterized in that the separating membrane (6) is in the shape of a pot.
 4. Pressure transmitter according to one of the foregoing claims, characterized in that the separating membrane (6) has a round cross section with a diameter (11) of less than 40 mm.
 5. Pressure transmitter according to one of the foregoing claims, characterized in that the base body (2) and/or the separating membrane (6) is/are at least partially made of a corrosion-resistant metal material, preferably high-grade steel.
 6. Pressure transmitter according to one of the foregoing claims, characterized in that the pressure-transmitting medium contains an oil, preferably a hydraulic oil or a silicone oil.
 7. Pressure transmitter according to one of the foregoing claims, characterized in that the base body (2) and/or the separating membrane (6) has/have temperature expansion coefficients that are smaller by a factor of 10 to 1000 than that of the pressure-transmitting medium.
 8. Pressure transmitter according to one of the foregoing claims, characterized in that the base body (2) and/or the separating membrane (6) has/have temperature expansion coefficient in the range of 10-100 10⁻⁶ K⁻⁶ for a temperature range of 20-100° C. 